Schmitt trigger with threshold voltage close to rail voltage

ABSTRACT

Voltage level shifting in a switching output stage is presented. The circuit may include a switching output stage configured to receive an analog input signal and provide a responsive digital output signal, the switching output stage having a first switching device coupled to a first supply voltage and a second switching device coupled to a second supply voltage, the first switching device and the second switching device being coupled to a common output node. The apparatus may also include a voltage level shifter circuit coupled to a switching control node of the second switching device, the voltage level shifter configured to shift a voltage level at the switching control node of the second switching device relative to the analog input signal, wherein the digital output signal at the common output node transitions as the input signal reaches a predetermined threshold value.

FIELD

This disclosure relates generally to a Schmitt trigger circuit, morespecifically, a Schmitt trigger circuit that has one of its thresholdvoltage very close to rail voltages

BACKGROUND

Switching amplifiers are common for many different applications, such aspulse generators. One application for a switching amplifier is in theaudio field. Often a switching amplifier is used as an Analog-to-DigitalConverter (ADC), or as a digital signal driver for an acoustic device,such as a speaker. One type of switching amplifier is commonly referredto as a class-D amplifier. Although various embodiments described hereinmay be discussed with relation to a class-D amplifier, one of ordinaryskill will recognize that the embodiments may be equally useful withother switching amplifier configurations.

A class-D amplifier is an electronic amplifier where internal powerdevices, such as Metal-On-Silicon Field-Effect-Transistors (MOSFETs),are operated as binary switches. The MOSFETs are often driven to beeither fully on or fully off. Ideally, zero time is spent transitioningbetween those two states. Class D amplifiers work by generating avariable duty cycle square wave of which the low-frequency portion ofthe spectrum nearly resembles the desired output signal, and of whichthe high-frequency portion serves no purpose other than to make thewaveform binary so it can be amplified by switching the power devices.

The switching amplifier often includes a PMOS transistor and an NMOStransistor. The PMOS and the NMOS may switch on and off respectively toprovide the switching output. Switching from one MOS to another needs tobe done carefully so that direct current from PMOS to NMOS should beminimized, and the non-overlap time during which both PMOS and NMOS areoff should also be minimized. It is also well known that electromagneticinterference (EMI) related to class D amplifier can be alleviated byslowing down the output slew rate.

To accurately control the PMOS and NMOS in a class D amplifier in amanner described above, it is desirable to use the amplifier output asfeedback to control the slew rate. Specifically, it is desirable todrive the gate of the power transistors with a weak strength during thetransition of output, but once output finishes its transition, it isdesired to drive the gate of the power transistors with full force sothat the overshoot, undershoot and energy loss all can be minimized. Asused herein, a “weak” strength MOSFET may include a device that is smallin physical dimensions relative to its complimentary device, or a devicewith lower charge-carrier mobility characteristics relative to itscomplimentary device. Prior art of using just a skewed inverter orconventional Schmitt trigger often cannot accurately control or robustlyset the threshold voltage very close to the rail voltage, which is wherethe switching truly finishes.

SUMMARY

Embodiments of systems, apparatuses, and methods for implementing aSchmitt trigger with threshold voltage close to rail voltage arepresented. In an embodiment, the apparatus includes a switching outputstage configured to receive an analog input signal and provide aresponsive digital output signal, the switching output stage having afirst switching device coupled to a first supply voltage and a secondswitching device coupled to a second supply voltage, the first switchingdevice and the second switching device being coupled to a common outputnode. The apparatus also includes a voltage level shifter circuitcoupled to a switching control node of the second switching device, thevoltage level shifter configured to shift a voltage level at theswitching control node of the second switching device relative to theinput signal, wherein the digital output signal at the common outputnode transitions as the input signal reaches a predetermined thresholdvalue that is very close to rail voltage. Additionally, the apparatusmay include a third switching device and a fourth switching device forimplementing hysteresis in the apparatus. Beneficially, such embodimentsmay shift the threshold of this Schmitt trigger toward the rail voltage.When the output of a class D amplifier is connected to the input of theembodiments of this Schmitt trigger, the exact moment when the class Damplifier finishes switching can be sensed. Therefore, overshoot and/orundershoot of the class D output can be reduced and the performance ofthe class D amplifier can be improved.

In an embodiment, the first supply voltage is higher than the secondsupply voltage. In such an embodiment, the voltage level shifter mayshift the input signal up by a predetermined shift value. Thus, thepredetermined threshold value is shifted toward the second supplyvoltage.

In another embodiment, the first supply voltage is lower than the secondsupply voltage. In such an embodiment, the voltage level shifter isconfigured to shift the input signal down by a predetermined shiftvalue, and thus the predetermined threshold voltage is shifted towardthe second supply voltage.

In one embodiment, the second switching device is stronger than thefirst switching device. As used herein a “strong” strength MOSFET mayinclude a device that is large in physical dimensions relative to itscomplimentary device, or a device with higher charge-carrier mobilitycharacteristics relative to its complimentary device. In anotherembodiment, a current supply drives the source node of the firstswitching device.

In an embodiment, the voltage level shifter includes a first diode and asecond diode, wherein an anode of the first diode is coupled to theinput signal and a cathode of the first diode is coupled to theswitching control node of the second switching device, and wherein ananode of the second diode is coupled to the switching control node ofthe second switching device and a cathode of the second diode is coupledto the input signal.

In another embodiment, the voltage level shifter includes a first levelshifter transistor, wherein a drain node and a gate node of the firstlevel shifter transistor are coupled to the input signal, and a sourcenode of the level shifter transistor is coupled to the switching controlnode of the second switching device, and a second level shiftertransistor, wherein a source node of the second level shifter transistoris coupled to the input signal, and a drain node and a gate node of thesecond level shifter transistor are coupled to the switching controlnode of the second switching device.

A method is also presented. In an embodiment, the method includesreceiving, in a switching output stage, an analog input signal. Themethod may also include generating a responsive digital output signal toa common output node of the switching output stage, the switching outputstage having a first switching device coupled to a first supply voltageand a second switching device coupled to a second supply voltage, thefirst switching device and the second switching device being coupled tothe common output node. Additionally, the method may include shifting avoltage level at a switching control node of the second switching devicerelative to the input signal with a voltage level shifter circuitcoupled to the switching control node of the second switching device,wherein the digital output signal at the common output node transitionsas the input signal reaches a predetermined threshold value that isclose to rail voltage.

A system is also presented. In an embodiment, the system includes anacoustical driver configured to convert an electrical signal intoacoustical energy. Additionally, the system may include a signal drivercoupled to the acoustical driver. The signal driver may include aswitching output stage configured to receive an analog input signal andprovide a responsive digital output signal, the switching output stagehaving a first switching device coupled to a first supply voltage and asecond switching device coupled to a second supply voltage, the firstswitching device and the second switching device being coupled to acommon output node. Additionally, the signal driver may include avoltage level shifter circuit coupled to a switching control node of thesecond switching device, the voltage level shifter configured to shift avoltage level at the switching control node of the second switchingdevice relative to the input signal, wherein the digital output signalat the common output node transitions as the input signal reaches apredetermined threshold value.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is illustrated by way of example and is/are notlimited by the accompanying figures, in which like references indicatesimilar elements. Elements in the figures are illustrated for simplicityand clarity and have not necessarily been drawn to scale.

FIG. 1 is a schematic block diagram illustrating one embodiment of asystem having a Schmitt trigger with threshold voltage close to railvoltage.

FIG. 2 is a schematic block diagram illustrating one embodiment of anapparatus having a Schmitt trigger with threshold voltage close to railvoltage.

FIG. 3 is a schematic block diagram illustrating another embodiment ofan apparatus having a Schmitt trigger with threshold voltage close torail voltage.

FIG. 4 is a schematic block diagram illustrating another embodiment ofan apparatus having a Schmitt trigger with threshold voltage close torail voltage.

FIG. 5 is a schematic block diagram illustrating another embodiment ofan apparatus having a Schmitt trigger with threshold voltage close torail voltage.

FIG. 6 is a flowchart diagram illustrating one embodiment of a methodfor implementing a Schmitt trigger with threshold voltage close to railvoltage.

DETAILED DESCRIPTION

The present embodiments provide for implementing a Schmitt trigger withthreshold voltage close to rail voltage. Embodiments of systems,apparatuses, and methods are described. Such embodiments may shift aninput signal level on one of the switching devices such that theswitching device turns on closer to the voltage rail coupled to theswitching device. For example, if the threshold of the Schmitt triggeris desired to be close to ground, then embodiment of this Schmitttrigger may include a weak PMOS transistor and a strong NMOS transistor,then the gate of the NMOS transistor may be coupled to the output of thelevel shifting circuit which shifts the input voltage up such that theNMOS transistor only switches off when the input signal is relativelyclose to ground. Since the NMOS transistor is much stronger than PMOStransistor, output will only toggle when the NMOS is fully turned off.Therefore, the threshold voltage in terms of input when output starts totoggle is shifted down. Beneficially, such embodiments may shift thethreshold toward the rail voltage. When the output of a class Damplifier is connected to the input of the embodiments of this Schmitttrigger, the exact moment when the class D amplifier finishes switchingcan be sensed. Therefore, overshoot and/or undershoot of the class Doutput can be reduced and the performance of the class D amplifier canbe improved.

FIG. 1 is a schematic block diagram illustrating one embodiment of asystem 100 for implementing a Schmitt trigger with threshold voltageclose to rail voltage. In an embodiment, the system 100 includes aSchmitt trigger device 102. The system 100 may also include a class Doutput stage 104. In an example embodiment, the class D output stage 104may include a first transistor and a second transistor coupled to a highand low voltage rail+Vdd. Additionally, each transistor may be driven bya pre-driver 106. The class D output stage 104 may receive an inputsignal from a pulse width modulator 108 which receives an input signal110. The Schmitt trigger circuit 102 may capture the moment when theoutput class D output stage 104 finishes switching from one rail voltageto the other rail voltage. The pre-driver 106 may use the output of theSchmitt trigger 102 to adjust the slew rate for the gate of thetransistors in the class D output stage 104. One of ordinary skill willrecognize alternative embodiments of a system 100 that may be used inassociation with the present embodiments.

FIG. 2 is a schematic block diagram illustrating one embodiment of aSchmitt trigger 102 with threshold voltage close to rail voltage.Circuit 200 may comprise an embodiment of the Schmitt trigger device 102described in FIG. 1. In an embodiment, the circuit 200 includes aswitching output stage 202. The switching output stage 202 may include afirst switching device 204 and a second switching device 206, eachsharing a common output node 208. The second switching device 206 may bemuch stronger than the first switching device 204. In an embodiment, thesecond switching device 206 may be coupled to the voltage level shiftercircuit 210. The voltage level shifter circuit 210 may be configured toshift a voltage level at a switching control node of the secondswitching device 206 relative to an input signal 214. For example, thevoltage level shifter circuit 210 may shift the input signal 214 by avoltage level that is very close to the threshold voltage of the secondswitching device 206. In such an embodiment, the voltage level shiftercircuit 210 may cause the digital output signal at the common outputnode 208 to transition as the input signal reaches a predeterminedthreshold, which is very close to ground. The third switching device 212may provide hysteresis in the circuit 200. Further examples of thestructure and functions of the voltage level shifter circuit 210 aredescribed in FIGS. 3-5.

FIG. 3 is a schematic block diagram illustrating another embodiment of acircuit 300 for implementing a Schmitt trigger with threshold voltageclose to rail voltage. In one embodiment, the circuit 300 may be used asSchmitt trigger device 102. In an embodiment, the circuit 300 includes aswitching output stage 202 and a voltage level shifter circuit 210 asdescribed in FIG. 2. In a further embodiment of the circuit 300, theswitching output stage 202 includes a first voltage rail 302, and asecond voltage rail 310. In the illustrated embodiment, the firstvoltage rail is Vdd, which may be a positive voltage supply, and thesecond voltage rail 310 may be ground. In a particular embodiment, thefirst voltage rail 302 may be at a higher magnitude potential level thanthe second voltage rail 310.

In an embodiment, the first switching device 204 for circuit 300 may bea first transistor device 304. Similarly, the second switching device206 for circuit 300 may be a second transistor device 306. Additionally,the third switching device 214 for circuit 300 may be a third transistordevice 314. In a further embodiment, the first transistor 304 and thesecond transistor 306 may be a Complimentary MOSFET pair, where thefirst transistor 304 is a PMOS device and the second transistor 306 isan NMOS device. In particular, the second transistor 306 may be muchstronger than the first transistor device 304. In such an embodiment,the common output node 308 may be coupled to the drain node of each ofthe first transistor 304 and the second transistor 306. Additionally, aninput signal may be received on input line 312 and coupled to a controlnode, such as the gate node, of each of the first transistor 304 and thethird transistor 314. The gate node of the second transistor 306 may becoupled to the voltage level shifter circuit 210.

The voltage level shifter circuit 210 may be coupled to the commonoutput node 308 as well as the input line 312. In a further embodiment,the voltage level shifter circuit 210 may be coupled between the inputnode 312 and the control or gate node of the second transistor 306,which may be an NMOS transistor in some embodiments. According to oneembodiment, the voltage level shifter circuit 210 may shift the voltagelevel up at the gate node of the second transistor 306 such that thetransistor is configured to switch off when the voltage at the inputsignal 312 is closer to ground than would be the case without thevoltage shifting. For example, if the second transistor 306 has athreshold voltage (Vth) of 0.6 V, then the voltage level shifter circuit210 may shift the input signal 312 up by around 0.6 V. In someembodiments, the voltage level shifter circuit 210 may shift the voltageof the input signal 312 up by about 0.1 V, 0.2 V, 0.3 V, 0.4 V, or 0.5V. One of ordinary skill will recognize that the voltage level shiftercircuit may shift the input signal more or less within the range of 0V-0.6 V, depending upon system configurations or performancerequirements. In a further embodiment, the circuit 300 may include athird transistor 314 and a fourth transistor 316 configured to providehysteresis. When the input 312 transitions from high to low, the thirdtransistor 314 may be turned off when the input signal 312 reaches justbelow the NMOS threshold voltage, while the second transistor 306 stillremains on. Therefore, the threshold voltage of the circuit 300 isdecided by the level shifter 210 and the second transistor 306. Once thecommon output 308 changes from low to high, the fourth transistor 316will turn on and strongly shut off the second transistor 306. When theinput 312 transitions start to change from low to high, initially output308 and the fourth transistor 316 will keep the second transistor fullyshut off, and the common output will be held high until the inputreaches the NMOS threshold voltage to turn on the third transistor 312.Therefore, the threshold for circuit 300 is very close to ground whenthe input transitions from high to low and the threshold is close to theNMOS threshold when input transitions from low to high.

FIG. 4 is a schematic block diagram illustrating another embodiment of acircuit 400 for implementing a Schmitt trigger with threshold voltageclose to rail voltage. In one embodiment, the circuit 400 may be used asSchmitt trigger device 102 as shown in FIG. 1. In an embodiment, thecircuit 400 includes the switching output stage 202 as described in FIG.3. The circuit 400 may also include a voltage level shifter circuit 210.In the depicted embodiment, the voltage level shifter circuit 210 mayinclude a diode or transistor 402 and a second diode or transistor 404arranged in a back-to-back or source-to-drain configuration.

For example, a drain node and a gate node of the first level shiftertransistor 402 are coupled to the gate node of the second transistor306, and a source node of the level shifter transistor 402 is coupled tothe input signal 312. A second level shifter transistor 404 is arrangedsuch that a gate node and a drain node of the second level shiftertransistor 404 is coupled to the input signal 312, and a source node ofthe transistor of second level shifter 404 is coupled to the switchingcontrol node (e.g., gate node) of the second transistor 306.

FIG. 5 is a schematic block diagram illustrating another embodiment of acircuit 500 for implementing a Schmitt trigger with threshold voltageclose to rail voltage. In one embodiment, the circuit 500 may be used asSchmitt trigger device 102. The circuit 500 of FIG. 5 is similar instructure and function to the circuit 400 of FIG. 4; however, the firstswitching transistor 304 is coupled to a current source 502 configuredto drive a small constant current I₁ to the source node of the firsttransistor 304 so that the driving force of the first transistor 304 islimited by the small current provided by current source 502.

It should be understood that the various operations described herein,particularly in connection with FIG. 6, may be implemented by processingcircuitry or other hardware components. The order in which eachoperation of a given method is performed may be changed, and variouselements of the systems illustrated herein may be added, reordered,combined, omitted, modified, etc. It is intended that this disclosureembrace all such modifications and changes and, accordingly, the abovedescription should be regarded in an illustrative rather than arestrictive sense.

FIG. 6 is a flowchart diagram illustrating one embodiment of a method600 for voltage level shifting in a switching output stage of a signalamplifier. In an embodiment, the method 600 includes receiving an analoginput signal at the input node 308 as shown at block 602. The switchingoutput stage 202 may generate a responsive digital output signal to acommon output node 208 of the switching output stage 202, as shown atblock 604. In an embodiment, the switching output stage 202 has a firstswitching device 204 coupled to a first supply voltage 302 and a secondswitching device 206 coupled to a second supply voltage 310. The firstswitching device 204 and the second switching device 206 may be coupledto the common output node 208. As shown at block 606, the voltage levelshifter circuit 210 may shift a voltage level at a switching controlnode of the second switching device 206 relative to the input signal308. In an embodiment, the digital output signal at the common outputnode 208 transitions as the input signal 312 reaches a predeterminedthreshold value.

Although this disclosure makes reference to specific embodiments,certain modifications and changes can be made to those embodiments.Accordingly, the specification and figures are to be regarded in anillustrative rather than a restrictive sense, and all such modificationsare intended to be included within the scope of this disclosure. Anybenefits, advantages, or solutions to problems that are described hereinwith regard to specific embodiments are not intended to be construed asa critical, required, or essential feature or element of any or all theclaims.

Unless stated otherwise, terms such as “first” and “second” are used toarbitrarily distinguish between the elements such terms describe. Thus,these terms are not necessarily intended to indicate temporal or otherprioritization of such elements. The terms “coupled” or “operablycoupled” are defined as connected, although not necessarily directly,and not necessarily mechanically. The terms “a” and “an” are defined asone or more unless stated otherwise. The terms “comprise” (and any formof comprise, such as “comprises” and “comprising”), “have” (and any formof have, such as “has” and “having”), “include” (and any form ofinclude, such as “includes” and “including”) and “contain” (and any formof contain, such as “contains” and “containing”) are open-ended linkingverbs. As a result, a system, device, or apparatus that “comprises,”“has,” “includes” or “contains” one or more elements possesses those oneor more elements but is not limited to possessing only those one or moreelements. Similarly, a method or process that “comprises,” “has,”“includes” or “contains” one or more operations possesses those one ormore operations but is not limited to possessing only those one or moreoperations.

1. An apparatus, comprising: a switching output stage configured toreceive an analog input signal and provide a responsive digital outputsignal, the switching output stage having a first switching devicecoupled to a first supply voltage and a second switching device coupledto a second supply voltage, the first switching device and the secondswitching device being coupled to a common output node; and a voltagelevel shifter circuit coupled to a switching control node of the secondswitching device, the voltage level shifter configured to shift avoltage level at the switching control node of the second switchingdevice relative to the analog input signal; wherein the digital outputsignal at the common output node transitions as the analog input signalreaches a predetermined threshold value.
 2. The apparatus of claim 1,wherein the first supply voltage is higher than the second supplyvoltage.
 3. The apparatus of claim 2, wherein the voltage level shifteris configured to shift the analog input signal up by a predeterminedshift value and wherein the predetermined threshold value is shiftedtoward the second supply voltage.
 4. The apparatus of claim 1, whereinthe first supply voltage is lower than the second supply voltage.
 5. Theapparatus of claim 4, wherein the voltage level shifter is configured toshift the analog input signal down by a predetermined shift value andwherein the predetermined threshold voltage is shifted toward the secondsupply voltage.
 6. The apparatus of claim 1, wherein the secondswitching device is stronger than the first switching device.
 7. Theapparatus of claim 1, wherein the voltage level shifter comprises afirst diode and a second diode, wherein an anode of the first diode iscoupled to the input signal and a cathode of the first diode is coupledto the switching control node of the second switching device, andwherein an anode of the second diode is coupled to the switching controlnode of the second switching device and a cathode of the second diode iscoupled to the analog input signal.
 8. The apparatus of claim 1, whereinthe voltage level shifter comprises: a first level shifter transistor,wherein a source node and a gate node of the first level shiftertransistor are coupled to the analog input signal, and a drain node ofthe level shifter transistor is coupled to the switching control node ofthe second switching device; and a second level shifter transistor,wherein a drain node of the second level shifter transistor is coupledto the analog input signal, and a source node and a gate node of thesecond level shifter transistor are coupled to the switching controlnode of the second switching device.
 9. The apparatus of claim 1,further comprising a third switching device having a control nodecoupled to the input signal, wherein the third switching device iscoupled between the second power supply and the common output.
 10. Theapparatus of claim 1, further comprising a fourth switching devicehaving a control node coupled to the common output node, wherein thefourth switching device is coupled between a control node of the secondswitching device and the second power supply.
 11. A method, comprising:receiving, in a switching output stage, an analog input signal;generating a responsive digital output signal to a common output node ofthe switching output stage, the switching output stage having a firstswitching device coupled to a first supply voltage and a secondswitching device coupled to a second supply voltage, the first switchingdevice and the second switching device being coupled to the commonoutput node; and shifting a voltage level at a switching control node ofthe second switching device relative to the analog input signal with avoltage level shifter circuit coupled to the switching control node ofthe second switching device; wherein the digital output signal at thecommon output node transitions as the analog input signal reaches apredetermined threshold value.
 12. The method of claim 11, wherein thefirst supply voltage is higher than the second supply voltage.
 13. Themethod of claim 12, wherein the voltage level shifter is configured toshift the analog input signal up by a predetermined shift value andwherein the predetermined threshold value is shifted toward the secondsupply voltage.
 14. The method of claim 11, wherein the first supplyvoltage is lower than the second supply voltage.
 15. The method of claim14, wherein the voltage level shifter is configured to shift the analoginput signal down by a predetermined shift value and wherein thepredetermined threshold voltage is shifted toward the second supplyvoltage.
 16. The method of claim 11, wherein the second switching deviceis stronger than the first switching device.
 17. The method of claim 11,wherein the voltage level shifter comprises a first diode and a seconddiode, wherein an anode of the first diode is coupled to the inputsignal and a cathode of the first diode is coupled to the switchingcontrol node of the second switching device, and wherein an anode of thesecond diode is coupled to the switching control node of the secondswitching device and a cathode of the second diode is coupled to theinput signal.
 18. The method of claim 11, wherein the voltage levelshifter comprises: a first level shifter transistor, wherein a sourcenode and a gate node of the first level shifter transistor are coupledto the input signal, and a drain node of the level shifter transistor iscoupled to the switching control node of the second switching device;and a second level shifter transistor, wherein a drain node of thesecond level shifter transistor is coupled to the input signal, and asource node and a gate node of the second level shifter transistor arecoupled to the switching control node of the second switching device.19. A system, comprising: an acoustical driver configured to convert anelectrical signal into acoustical energy; and a signal driver coupled tothe acoustical driver, the signal driver comprising: a switching outputstage configured to receive an analog input signal and provide aresponsive digital output signal, the switching output stage having afirst switching device coupled to a first supply voltage and a secondswitching device coupled to a second supply voltage, the first switchingdevice and the second switching device being coupled to a common outputnode; and a voltage level shifter circuit coupled to a switching controlnode of the second switching device, the voltage level shifterconfigured to shift a voltage level at the switching control node of thesecond switching device relative to the analog input signal; wherein thedigital output signal at the common output node transitions as theanalog input signal reaches a predetermined threshold value.
 20. Thesystem of claim 19, wherein the first supply voltage is higher than thesecond supply voltage.
 21. The system of claim 20, wherein the voltagelevel shifter is configured to shift the analog input signal up by apredetermined shift value and wherein the predetermined threshold valueis shifted toward the second supply voltage.
 22. The system of claim 19,wherein the first supply voltage is lower than the second supplyvoltage.
 23. The system of claim 22, wherein the voltage level shifteris configured to shift the analog input signal down by a predeterminedshift value and wherein the predetermined threshold voltage is shiftedtoward the second supply voltage.
 24. The system of claim 19, whereinthe second switching device is stronger than the first switching device.25. The system of claim 19, wherein the voltage level shifter comprisesa first diode and a second diode, wherein an anode of the first diode iscoupled to the input signal and a cathode of the first diode is coupledto the switching control node of the second switching device, andwherein an anode of the second diode is coupled to the switching controlnode of the second switching device and a cathode of the second diode iscoupled to the analog input signal.
 26. The system of claim 19, whereinthe voltage level shifter comprises: a first level shifter transistor,wherein a source node and a gate node of the first level shiftertransistor are coupled to the analog input signal, and a drain node ofthe level shifter transistor is coupled to the switching control node ofthe second switching device; and a second level shifter transistor,wherein a drain node of the second level shifter transistor is coupledto the analog input signal, and a source node and a gate node of thesecond level shifter transistor are coupled to the switching controlnode of the second switching device.
 27. The system of claim 19, furthercomprising a third switching device having a control node coupled to theinput signal, wherein the third switching device is coupled between thesecond power supply and the common output.
 28. The system of claim 19,further comprising a fourth switching device having a control nodecoupled to the common output node, wherein the fourth switching deviceis coupled between a control node of the second switching device and thesecond power supply.